1. Field of the Invention
The present invention relates to a semiconductor device
Priority is claimed on Japanese Patent Application No. 2010-200004, filed Sep. 7, 2010, the content of which is incorporated herein by reference.
2. Description of the Related Art
PRAM (phase change random access memory) stores data by means of a change in resistance upon phase transition of a phase-change memory material layer between a crystalline state and an amorphous state. A memory cell circuit of PRAM has a structure in which one transistor and one resistor are included in one cell, as shown in FIG. 39 (see, for example, S. M. Sadeghipour, L. Pileggi, M. Asheghi, “Phase Change Random Access Memory, Thermal Analysis”, ITHERM-06, pp 660).
As shown in FIG. 40, a general PRAM cell 100 with respect to one resistor has a cylindrical multi-layered structure including: a heater electrode 101; a phase-change memory material film 102; an upper electrode 103; and an insulating film 104 surrounding the heater electrode 101. The upper electrode 103 is larger in diameter than the heater electrode 101. Phase transition between the crystalline state and the amorphous state is implemented as follows. When a current is provided to the heater electrode 101, heat from the heater electrode 101 transfers to the phase-change memory material layer 102. Thus, phase transition occurs, and a phase-change region 102a is formed in the phase-change memory material layer 102. The current required for causing the phase transition is called a rewriting current.
Recently, it has been required to reduce the amount of the rewriting current to achieve lower power consumption. In the case of FIG. 40, a heat q1 is supplied from the heater electrode 101 to the phase-change memory material layer 102. A heat q2 is used for increasing a temperature of the heater electrode 101. Thus, a phase-change region 102a is formed over the heater electrode 101. In this case, not only the heat q1, but also the following heats q3, q4, q5, and q6 are supplied from the heater electrode 101. The heat q3 diffuses into the insulating film 104. The heat q4 diffuses into the heater electrode 101. The heats q5 and q6 spread from the phase-change region 102a toward the upper electrode 103 and the phase-change memory material layer 102.
A thermal conductivity of the heater electrode 101 is 20 W/k·m, which is much greater than that of another phase-change memory element, such as the phase-change memory material layer 102. For this reason, the heat q4, which is approximately 60 to 70% of the total heat generated from the heater electrode 101, diffuses into the heater electrode 101 itself rather than diffusing into the phase-change memory material layer 102.
Additionally, the thermal conductivities of the phase-change memory material layer 102, the upper electrode 103, and the insulating film 104 are smaller than that of the heater electrode 101, but are greater than that of the phase-change region 102a. For this reason, the heat generated from the heater electrode 101 spreads toward the phase-change memory material layer 102, the upper electrode 103, and the insulating film 104. Consequently, only the heat q1, which is only around 1% of the entire heat generated from the heat electrode 101, diffuses into the phase-change memory material layer 102 to contribute to the phase transition, thereby lowering the thermal efficiency, and therefore requiring a large rewriting current.
To achieve a reduction in the amount of rewriting current, the aforementioned document discloses a PRAM cell 200 as shown in FIG. 41, which prevents the heat of the heater electrode 101 from diffusing the heater electrode 101 itself. Similar to the PRAM cell 100 shown in FIG. 40, the PRAM cell 200 has a cylindrical multi-layered structure including a heater electrode 201, a phase-change memory material layer 202, an upper electrode 203, and an insulating film 204. The PRAM cell 200 differs from the PRAM cell 100 in that the diameter of the heater electrode 201 is equal to that of the upper electrode 203, and that the insulating film 204 surrounds a phase-change region 202a formed in the phase-change memory material film 202.
In the case of the PRAM cell 200, a current concentrates only in the phase-change memory material film 202. For this reason, the phase change region 202a is formed in a middle portion of the phase-change memory material film 202, which is separated from the heater electrode 201 having the large thermal conductivity. Accordingly, the thermal efficiency of the PRAM cell 200 becomes higher than that of the PRAM cell 100, and thereby a reduction in the rewriting current is expected (see also M. Gill, T. Lowrey, and J. Park, “Ovonic Unified Memory—A High-performance Nonvolatile Memory Technology for Stand Alone Memory and Embedded Applications”, ISSCC 2002 Digest of Technical Papers vol. 45, pp. 202-203 and 459, February 2002; and G. Servalli, “A 45 nm Generation Phase Change Memory Technology”, IEDM-09, pp. 113-116).
However, in the case of the PRAM cell 200 shown in FIG. 41, prevention of heat from diffusing into the lower electrode 201 can be achieved, but prevention of heat from diffusing into the insulating film 204 cannot be achieved. For this reason, the larger amount of heat causing phase transition than in the case of the PRAM cell 100 can be secured, but the amount of heat diffusing into the phase-change memory material film 202 is still small. Accordingly, further enhancement of thermal efficiency and further reduction in the rewriting current are required.